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As compared to RIP and SAP, NLSP provides improved routing, better efficiency, and scalability. In addition, NLSP-based routers are backward-compatible with RIP-based routers. NLSP-based routers use a reliable delivery protocol, so delivery is guaranteed. Furthermore, NLSP facilitates improved routing decisions because NLSP-based routers store a complete map of the network, not just next-hop information such as RIP-based routers use. Routing information is transmitted only when the topology has changed, not every 60 seconds as RIP-based routers do, regardless of whether the topology has changed. Additionally, NLSP-based routers send service-information updates only when services change, not every 60 seconds as SAP does.
NLSP is efficient in several ways. It is particularly useful over a WAN link because its support of IPX header compression makes it possible to reduce the size of packets. NLSP also supports multicast addressing so that routing information is sent only to other NLSP routers, not to all devices as RIP does.
In addition, NLSP supports load balancing across parallel paths and improves link integrity. It periodically checks links for connectivity and for the data integrity of routing information. If a link fails, NLSP switches to an alternate link and updates the network-topology databases stored in each node when connectivity changes occur anywhere in the routing area.
In terms of scalability, NLSP can support up to 127 hops (RIP supports only 15 hops) and permits hierarchical addressing of network nodes, which allows networks to contain thousands of LANs and servers.
NLSP supports hierarchal routing with area, domain, and global internetwork components. An area is a collection of connected networks that all have the same area address. A domain is a collection of areas that belong to the same organization. A global internetwork is a collection of domains that usually belong to different organizations but with an arms-length relationship. Areas can be linked to create routing domains, and domains can be linked to create a global internetwork.
NLSP supports three levels of hierarchical routing: Level 1, Level 2, and Level 3 routing. A Level 1 router connects network segments within a given routing area. A Level 2 router connects areas and also acts as a Level 1 router within its own area. A Level 3 router connects domains and also acts as a Level 2 router within its own domain. Figure 40-1 illustrates the three routing levels NLSP defines.
Hierarchical routing simplifies the process of enlarging a network by reducing the amount of information that every router must store and process to route packets within a domain. A Level 1 router is required to keep detailed information only about its own area instead of storing link-state information for every router and network segment in its domain. To exchange traffic with other areas, a Level 1 router must only find the nearest Level 2 router. Between areas, Level 2 routers advertise the area address(es) only for their respective areas, not their entire link-state databases. Level 3 routers perform similarly between domains.
Adjacency-establishment procedures vary depending upon whether the router is establishing and maintaining adjacencies over a WAN or a LAN.
Establishing router adjacency over a WAN involves first establishing the underlying data-link connection (details depend upon the medium). The routers then exchange identities by using the IPX WAN Version 2 protocol and determine certain operational characteristics of the link. Hello packets are exchanged and the routers update their adjacency databases. The routers then exchange both link-state packets (LSPs) describing the state of their links and IPX data packets over the link. To maintain a WAN link, the router maintains a state variable indicating whether the link is up, down, or initializing for each adjacency. If the router does not hear from a neighbor within the time specified in a holding timer, the router generates a message indicating that the link is down and deletes the adjacency.
WAN Hello packets enable routers to discover each other's identity, decide whether they are in the same routing area, and determine whether other routers and links are operational. A router sends Hello packets when the circuit is first established, when a timer expires, or when the contents of the next Hello to be transmitted are different than the contents of the previous Hello transmitted by this system (and one or more seconds have elapsed since the previous Hello). Hello packets are sent as long as the circuit exists.
A typical startup procedure between two routers (A and B) on a WAN link begins with the link in the down state. Router A sends a WAN Hello indicating the down state to Router B, which changes its state for the link to Initializing. Router B sends a WAN Hello with a field indicating Initializing to Router A. Router A then changes its state for the link to initializing and sends a WAN Hello with a field indicating this to Router B. Router B changes its state for the link to the up state and sends a WAN Hello with a field indicating its new state. Finally, Router A changes its state for the link to Up.
When a broadcast circuit, such as an 802.3 Ethernet and 802.5 Token Ring, is enabled on a router, the router begins sending and accepting Hello packets from other routers on the LAN and starts the Designated Router election process.
Periodically, every router sends a multicast Hello packet on the LAN. The router with the highest priority (a configurable parameter) becomes the Level 1 Designated Router on the LAN. In case of a tie, the router with the higher MAC address wins.
Hello packets enable routers on the broadcast circuit to discover the identity of the other Level 1 routers in the same routing area on that circuit. The packets are sent immediately when any circuit has been enabled to a special multicast destination address. Routers listen on this address for arriving Hello packets.
An NLSP router extracts certain information from the adjacency database and adds locally derived information. Using this information, the router constructs a link-state packet (LSP) that describes its immediate neighbors. All LSPs constructed by all routers in the routing area make up the link-state database for the area.
The NLSP specification intends that each router maintain a copy of the link-state database and keep these copies synchronized with each other. The link-state database is synchronized by reliably propagating LSPs throughout the routing area when a router observes a topology change. Two methods ensure that accurate topology-change information is propagated: flooding and receipt confirmation.
Flooding is instigated when a router detects a topology change. When such a change is detected, the router constructs a new LSP and transmits it to each of its neighbors. Such LSPs are directed packets on a WAN and multicast packets on a LAN. Upon receiving an LSP, the router uses the sequence number in the packet to decide whether the packet is newer than the current copy stored in its database. If it is a newer LSP, the router retransmits it to all its neighbors (except on the circuit over which the LSP was received).
The receipt-confirmation process is different for LANs and WANs. On WANs, a router receiving an LSP replies with an acknowledgment. On LANs, no explicit acknowledgment occurs, but the Designated Router periodically multicasts a packet called a complete sequence number packet (CSNP) that contains all the LSP identifiers and sequence numbers it has in its database for the entire area. This ensures that other routers can detect whether they are out of synchronization with the Designated Router.
NLSP supports a hierarchical addressing scheme. Each routing area is identified by two 32-bit quantities: a network address and a mask. This pair of numbers is called an area address. Expressed in hexadecimal, an example of an area address follows:
In the example area address above, the first 24 bits (012345) identify the routing area. The remaining 8 bits are used to identify individual network numbers within the routing area (for example, 012345AB, 012345C1, 01234511). Figure 40-2 highlights the above addressing concepts with three different networks in a single area.
A routing area can have as many as three different area addresses, each with a different mask. Having more than one area address allows the routing area to be reorganized without interrupting operations. Any combination of area addresses can be used within a domain.
Two types of NLSP Hello packets exist: WAN Hello and Level 1 LAN Hello packets.
Figure 40-3 illustrates the fields of a WAN Hello packet.
The following WAN hello packet field descriptions summarize each field illustrated in Figure 40-3.
Figure 40-4 illustrates the fields of a LAN Level 1 Hello packet.
The following Level 1 LAN Hello packet field descriptions summarize each field illustrated in Figure 40-4:
Posted: Thu Jun 17 16:25:59 PDT 1999
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